Reference is made to U.S. Pat. No. 5,608,514, (the '514 patent) issued Mar. 4, 1997 to the same inventors as the present application, assigned to the same assignee as the present application, and incorporated herein by reference. The '514 patent describes a technique for achieving high range resolution for a single pixel ladar by employing frequency modulation (FM) radar ranging principles.
Three-dimensional imaging of a scene is achieved by mechanically scanning the single pixel ladar or by building arrays of such ladars. To perform ranging on a single pixel, the ladar's laser transmitter is amplitude modulated with a radio-frequency subcarrier which itself is linearly frequency modulated. The target-reflected light is incoherently detected with photodiode and converted into a voltage waveform.
The voltage waveform is then mixed with an undelayed sample of the original laser amplitude modulation waveform. The output of the mixer is processed to remove "self clutter" that is commonly generated in FM ranging systems and obscures the true target signals. The clutter-free mixer output is then Fourier transformed to recover target range.
The '514 patent teaches the use of a single pixel ladar and does not teach or suggest the use of focal plane detector arrays similar to ones used in television cameras. Such focal plane detector arrays may be incorporated into a ladar to image scenes without mechanical scanning. "Characterization of a Scannerless Ladar System", Sandia National Laboratories, December, 1993, and Scott, U.S. Pat. No. 4,935,616, issued Jun. 19, 1990, both of which are incorporated herein by reference, describe a scannerless ladar system with a gain-modulated micro-channel plate image intensifier (one form of an electro-optic modulator) in front of a focal plane detector array. In such a system, the illumination source is an array of light emitting diodes which are amplitude modulated with a constant frequency sinusoid.
The same modulation as applied to the light emitting diodes is applied to the image intensifier. The intensifier effectively controls the amount of light reaching the detector array as a function of the relative phase between the modulation on the received light and the modulation applied to the intensifier. In the most basic sense, the signal processor maps pixel amplitude to this relative phase which is then used to estimate range to the target.
The Sandia Labs and Scott ladar systems will yield range ambiguities which are periodic with half of the wavelength of the modulation. The Sandia Labs and Scott ladar systems do not employ frequency modulation of the AM rate nor does it employ FM radar range-gate formation algorithms. As a result, range resolution may be limited.